The present invention relates to a grinding method wherein good grinding quality is attained with higher surface speed of the grinding wheels.
FIG. 1 is a graph showing grinding quality experimented in spark-out grinding by an internal grinding wheel which has a fine and easy-bending arbor, with the grinding wheel revolution speed being held as a varying parameter and the workpiece revolution speed constant. The graph illustrates, the initial depth of cut, that is, the amount of the grinding wheel surface penetration into the workpiece, at the initial state of the spark-out grinding, assuming no bending of the grinding wheel arbor but actually the amount of the grinding wheel arbor bending is 0.1 mm, which is called residual stock removal. In case of 10,000 r.p.m. of the wheel revolution speed, 0.1 mm is the residual stock removal remained through a lapse of time. In case of 30,000 r.p.m., the residual stock removal was reduced in an exponential curve and ws saturated, after 35 seconds, in 35 .mu.m of the residual stock removal, depth of 65 .mu.m being ground with the arbor bending force. In case of 60,000 r.p.m. saturated, after 25 seconds, in 10 .mu.m of the residual stock removal.
This experiment clearly shows the fact that higher surface speed of the grinding wheel gives better grinding quality.
It has also been found out that grinding quality depends on the ratio of grinding wheel surface speed to workpiece surface speed, that is, the best grinding quality or sharpness in a grinding wheel surface speed is attained by selecting a workpiece surface speed which gives approximately ratio of 0.1 to the grinding wheel surface speed, other values of the workpiece surface speed, usually in far lower case, giving not so good grinding quality. Under good grinding quality, grinding process is able to correct such initial surface errors on the workpiece as taper and distortion in a section, and to prevent taper error mainly due to the grinding wheel arbor bending or sectional distortion error, which usually occurs, in case of grinding a workpiece having some grooves or notches on the grinding surface, in a manner that the groove allows the grinding wheel to more easily approach the workpiece when the groove passes the intersection of the grinding wheel and the workpiece, and the adjacent area of the workpiece surface to the groove is, thereby, ground a little depressed.
Other ratios of the workpiece surface speed and the grinding wheel speed of poor grinding quality are useful and sometimes actually used for good surface roughness such as mirror finishing.
It is also well known that, even if the ratio of grinding wheel surface speed to workpiece surface speed is best suited for grinding quality, only poor grinding quality would be attained if the revolution of the grinding wheel spindle or the work spindle, especially the former in internal grinding which has a fine arbor for grinding deep and small diameter hollows, corresponds to its resonant frequency. The spindle or its grinding wheel arbor should be rotated at a revolution speed sufficiently higher or lower than the resonant speed thereby avoiding the correspondence to this resonant frequency. A lower speed is safer as the spindle or its arbor does not pass through its resonant frequency during the running up and down.
From the results of our experiments, it is found that the resonant frequency of an internal grinding arbor which has a grinding wheel on the nose shifts according to the wheel contact condition with a workpiece or a dresser. As shown in FIG. 2, an internal grinding wheel arbor of 50 mm length and 6 mm diameter has a resonance-peak value R, at about 48,000 r.p.m. in its speed when it is free from grinding, being supported in a cantilever state, which is almost the same as in its dressing condition in which it is in slight contact with the dresser. When 200 gr. load is imposed on the grinding wheel in a radial direction, making its supporting condition a two point support, which is almost equivalent to its grinding state, the resonance-peak value is shifted from R, to R', of about 120,000 r.p.m. in its speed.
In spark-out operation wherein the mean load of 20 gr. is observed, the resonance-peak-value alternately appears at 48,000 r.p.m. or 120,000 r.p.m. according to each-moment contact condition of the wheel to the work.
Thus, resonance frequencies of the grinding wheel shifts up and down in response to grinding step changes, the lowest resonance frequency appearing in dressing or spark-out operation.
Therefore, in the conventional grinding practice, the wheel revolution speed is usually limited lower than the resonance speed in dressing or spark-out operation, from which results poor grinding quality during the infeed grinding step as well as inferior grinding efficiency. BRIEF SUMMARY OF THE INVENTION
It is the principal object of this invention to provide grinding method by which workpieces are ground very accurate with higher efficiency.
It is another object of the invention to provide grinding method suitable in particular for grinding small-sized and precision workpieces.
These and other objects are attained by grinding method in which a grinding wheel revolution speed is set for feed grinding at a proper high value apart from that corresponding to a first resonance frequency of the feedgrinding wheel spindle, this value of the revolution being stable and suitable to efficiently grind workpieces with high grinding quality, and when spark-out or finish grinding signal is detected, the speed is lowered to another value different from that corresponding to a second resonance frequency of the wheel spindle under spark-out or similar operation, this another value unsuitable for efficient and high quality grinding but suitable to improve workpiece surface roughness with some polishing function.